Hello Diego,
You may try to purge the old jobs.
In general the LMS 4.2 runs VRFcollection jobs and Summarizer jobs periodically. When you hit a high number of jobs (15K if I'm not wrong), you start to get an error message on Job browser page. Then the purging may come in handy.
Go to [Admin] > [Network] > [Purge Settings].
To purge the jobs related to VRF, see the topic "Purging VRF Management Reports Jobs and Archived Reports" on the following document.
http://www.cisco.com/en/US/docs/net_mgmt/ciscoworks_lan_management_solution/4.2/user/guide/admin/purgeset.pdf
To purge Summarizer jobs, take a look at the topic "Performance Purge Jobs" from the same document.
Let me know if it helped.